1. Field of the Invention
The present invention relates to an electric motor used for railway vehicles such as the electric car, and more particularly, it relates to an air-cooled type induction motor capable of taking air into the motor to cool the motor.
2. Description of the Related Art
Some of the air-cooled type motors are disclosed in a chapter titled "On The Induction Motor For Use With Vehicles" in the magazine "SCIENCE OF ELECTRIC CARS", February edition, 1988, pages 18-23, in published Unexamined Japanese Patent Application No. 61-94545, and also Utility Model Application No. 62-11371. As apparent from these references, the squirrel-cage type induction motor which can be controlled by VVVF inverters is now being used as the main motor for railway vehicles such as the electric train. The induction motor has no brushes. This enables the motor to require less maintenance, rotated at a higher speed, and made smaller in size, lighter in weight and larger in capacity.
The air-cooled squirrel-cage type induction motor includes a stator and a rotor. The stator includes stator core, coils and the like. The stator core is fixed to the inner circumference of the housing or frame of the motor. The stator coils are incorporated into inner circumferential portions of the stator core. Further, bearings for supporting the rotor in a smoothly rotatable state are arranged around the center of each of both end plates which form a part of the frame and which hold the rotor between them. The rotor includes a rotor shaft, a rotor core, a plurality of rotor bars, two end rings and the like. Both ends of the rotor shaft are supported rotatable by bearings The rotor core is fixed to the rotor shaft. The plurality of rotor bars are incorporated into the outer circumferential portion of the rotor core. Both ends of each of these rotor bars are connected to the end rings.
In the case of the air-cooled motor having the above-described structure, a moving magnetic field is generated from the stator coils when current is supplied to them. When this moving magnetic field crosses the rotor bars, an electromotive force is derived. The rotor is rotated by interaction between the current and magnetic field generated.
In the case of this air-cooled motor, a large amount of heat is generated from the stator and the rotor when the motor is being rotated. When the stator coils and the rotor bars are heated by this heat to temperatures higher than predetermined ones, the insulating capacity of the stator coils is deteriorated while the strength of material of which the rotor bars are made is lowered. In order to prevent this, outside air is taken into the motor to cool its inside when it is being rotated.
This air-cooled motor is provided with an air inlet port at one end thereof in the axial line around which the stator is rotated. Further, an air outlet port is formed at the other end of the motor. Furthermore, a gap is formed between the stator core and the rotor core, and a plurality of ventilating holes are formed in the rotor core in the rotor, extending along the axial line around which the rotor core is rotated.
The induction motors of this kind are grouped into the ones of the forcedly-cooling type in which cooling air is forcedly fed from outside into the frame through the air inlet port, using a fan located outside the motor, and the ones of the induced type in which an impeller fixed to the rotor shaft is located in the frame and rotated together with the rotor to take the cooling air into the frame through the air inlet port.
The cooling air introduced into the frame is taken into one end of the stator core and blown out of the other end thereof, passing through the plurality of ventilating holes. This cooling air is finally exhausted outside the frame through the air outlet. The stator coils, the rotor bars and the like in the frame are cooled by the cooling air which passes through the frame.
The above-described induction motor is provided with a plurality of narrow ventilating holes through which the cooling air is circulated. Sound waves (or noise) having an uncertain frequency are generated by the cooling air thus circulated. The frequency of the sound waves changes as the rotation number of the motor is increased (the vehicle is accelerated) or decreased (the vehicle is decelerated). This noise is harsh to the ears of the human being and it makes passengers in the vehicle feel uncomfortable. When the motor is used for railway trains, for example, the noise is feared to cause a public nuisance to those people who live along the railroad. Further, in the case where the noise is increased when the motor comes to or near to its usually-used rotation number, the vehicles cannot be practically run on roads or railroads.